1. Field of the Invention
The present invention relates generally to the field of oxygen scavenging materials. More particularly, it concerns vinyl crosslinked oxygen scavenging compositions and methods and components for producing the same.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition, such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking. In the packaging industry, several means for limiting oxygen exposure have already been developed, including modified atmosphere packaging (MAP), vacuum packaging and oxygen barrier film packaging.
Another means for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. Incorporation of a scavenger in the package can provide a uniform scavenging effect throughout the package. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it is passes through the walls of the package, thereby maintaining the lowest possible oxygen level throughout the package.
While much emphasis is being placed on thermoplastic oxygen scavenging compositions for use in packaging, it would also be desirable to have oxygen scavenging compositions that have a thermoset structure. Such thermoset compositions could permit oxygen scavenging technology to be used in certain contexts (e.g. coatings) in which it would be difficult to use a thermoplastic composition. Furthermore, oxygen scavenging compositions that tend to have inherent barrier properties are also desirable. It would also be beneficial to have oxygen scavenging coatings that can both protect a metal container (e.g. a can) from corrosion, while conferring oxygen scavenging benefits to the oxygen-sensitive contents of the container.
The present invention is directed to vinyl crosslinked oxygen scavenging compositions, oxygen scavenging polymers that can be reacted with vinyl crosslinkers, and methods of preparing vinyl crosslinked oxygen scavenging coatings.
One aspect of the invention is an oxygen scavenging composition that comprises at least one oxygen scavenging polymer and at least one vinyl crosslinker. The oxygen scavenging polymer comprises a polymeric backbone, at least two backbone crosslinking functional groups that are carbon-carbon double bonds in the backbone and at least one cyclic organic moiety having from 5 to 9 carbon atoms in its ring, and at least one carbon-carbon double bond in its ring. The backbone crosslinking groups are capable of reacting with the vinyl crosslinker. Preferably the backbone carbon-carbon double bonds capable of being crosslinked are in the trans configuration. The vinyl crosslinker comprises at least two functional groups that are capable of reacting with the crosslinking functional groups of the oxygen scavenging polymer. Vinyl crosslinkers that can be used include styrene, divinyl benzene, methyl acrylate, cyclohexenyl methyl acrylate, cyclohexenyl methyl methacrylate, and methyl acrylate, among others.
Cyclohexenyl methyl acrylate or cyclohexenyl methyl methacrylate crosslinkers comprise a cyclic organic oxygen scavenging moiety (e.g. cyclohexenyl). Another embodiment of the present invention is an oxygen scavenging composition that comprises a low molecular weight polymer and at least one oxygen scavenging vinyl crosslinker selected from cyclohexenyl methyl acrylate and cyclohexenyl methyl methacrylate, wherein the low molecular weight polymer comprises a polymeric backbone and at least two backbone crosslinking functional groups that are carbon-carbon double bonds (preferably trans double bonds)in the backbone, wherein the low molecular weight polymer has a number average molecular weight between about 500 Mn and about 5000 Mn, and an acid number of between about 5 mg KOH/g and 50 mg KOH/g. Thus, when cyclohexenyl methyl acrylate or cyclohexenyl methyl methacrylate crosslinkers are used in preparing crosslinked oxygen scavenging compositions from a low molecular weight polymer that does not comprise cyclic organic oxygen scavenging moieties, cyclic organic oxygen scavenging moieties (e.g. cyclohexenyl groups) can be introduced into the crosslinked product by the crosslinker.
The present invention involves crosslinking between the backbones of oxygen scavenging polymers. It is preferred that only the backbone carbon-carbon double bonds, (which in certain embodiments of the present invention can be derived from maleic anhydride or fumaric acid) participate in the crosslinking reaction, and that the oxygen scavenging cyclic moiety, particularly its ring double bond, does not participate in the crosslinking reaction. Furthermore it is preferred that oxygen scavenging carried out by compositions of the present invention is such that the chemical reaction does not result in the cleavage of the backbone of the crosslinked oxygen scavenging polymer, or in the release of compounds that can have undesirable organoleptic effects. Furthermore, the composition can be in the form of a coating or an adhesive.
Another aspect of the invention is a method of preparing an oxygen scavenging coating. A coating can be prepared comprising at least one oxygen scavenging polymer and at least one vinyl crosslinker, wherein both the polymer and vinyl crosslinker are as described above. Alternatively, a coating can be prepared comprising at least one low molecular weight polymer and at least one oxygen scavenging vinyl crosslinker, wherein both the polymer and crosslinker are as described above. The vinyl crosslinker can serve both as the monomeric crosslinker and as the solvent in preparing the coating. In certain cases, when neither the vinyl crosslinker nor the polymer is a liquid, the coating formulation can comprise a solvent. The coating formulation is applied to a surface and is then cured. The coating formulation is cured to form a thermoset material. In certain embodiments in which the coating formulation further comprises an oxygen scavenging initiator, the method can further comprise initiating oxygen scavenging.
Still another aspect of the invention is an oxygen scavenging composition comprised of at least one crosslinked oxygen scavenging polymer. The crosslinked oxygen scavenging polymer comprises a polymeric backbone, at least one cyclic organic moiety having from 5 to 9 carbon atoms and at least one carbon-carbon double bond in its ring. The crosslinked polymer is the product of a reaction of at least one low molecular weight oxygen scavenging polymer and at least one vinyl crosslinker, the vinyl crosslinker is as described above. The low molecular weight oxygen scavenging polymer has a number average molecular weight of between about 500 Mn and about 5000 Mn, and comprises a polymeric backbone having at least two backbone crosslinking functional groups that are carbon-carbon double bonds in the backbone and a cyclic organic moiety which is the same as is present in the crosslinked oxygen scavenging polymer. Preferably, the backbone crosslinking functional groups are trans configuration carbon-carbon double bonds. The composition can further comprise one or more additional components, as described above.
Another aspect of the invention is an oxygen scavenging polymer, capable of being crosslinked, that is the product of a reaction comprising maleic anhydride and at least one diol. This oxygen scavenging polymer comprises a polymeric backbone and at least two backbone crosslinking functional groups, as described above. It has a number average molecular weight between about 500 Mn and about 5000 Mn. Furthermore, the oxygen scavenging polymer comprises at least one cyclic organic moiety having a formula selected from formula (I), (II) (III), and (IV). Formula (I) is 
wherein Q is xe2x80x94(CR2R3)nxe2x80x94, n is an integer from 0 to 3, inclusive; wherein Z is xe2x80x94(CR4R5)exe2x80x94, e is an integer from 0 to 3, inclusive; n+exe2x89xa63; wherein q0, q1, q2, q3, q4, r, each R2, each R3, each R4, and each R5 are independently selected from hydrogen, linear C1-C20 alkyls, branched C1-C20 alkyls, cyclic C1-C20 alkyls, polycyclic C1-C20 alkyls, aromatic groups, halogens, and sulfur-containing substituents, and when r is hydrogen at least one of q1, q2, q3, and q4is hydrogen. Preferably, q0, q1, q2, q3, q4, r, each R2, each R3, each R4, and each R5 are independently selected from hydrogen, methyl and ethyl. Formula (II) is 
wherein r1, r2, r3, r4, and r5 are independently selected from hydrogen, linear C1-C20 alkyls, branched C1-C20 alkyls, cyclic C1-C20 alkyls, polycyclic C1-C20 alkyls, aromatic groups, halogens, and sulfur-containing substituents. Preferably, r1, r2, r3, r4, and r5 are independently selected from hydrogen, methyl, and ethyl. Formula (III) is 
wherein A is xe2x80x94(CR5R6)gxe2x80x94, g is an integer from 0 to 3, inclusive; wherein E is xe2x80x94(CR7R8)pxe2x80x94, p is an integer from 0 to 3, inclusive; g+pxe2x89xa63; wherein q6, q7, q8, q9, rxe2x80x2, each R5, each R6, each R7, and each R8 are independently selected from hydrogen, linear C1-C20 alkyls, branched C1-C20 alkyls, cyclic C1-C20 alkyls, polycyclic C1-C20 alkyls, aromatic groups, halogens, and sulfur-containing substituents, and when rxe2x80x2 is hydrogen, at least one of q6, q7, q8, and q9 is hydrogen. Preferably, q6, q7, q8, q9, rxe2x80x2, each R5, each R6, each R7, each R8 are independently selected from hydrogen, methyl and ethyl. Formula (IV) is 
wherein m is xe2x80x94(CR9R10)h(CR11R12)jxe2x80x94, wherein the ring carbon adjacent to the double bond is bonded to R9 and R10, h is an integer from 0 to 1, inclusive, j is an integer from 0 to 3, inclusive, and when h+jxe2x89xa71, h is 1; wherein q10, q11, q12, q13, rxe2x80x3, R9, R10, each R11, and each R12 are independently selected hydrogen, linear C1-C20 alkyls, branched C1-C20 alkyls, cyclic C1-C20 alkyls, polycyclic C1-C20 alkyls, aromatic groups, halogens, and sulfur-containing substituents; and wherein when rxe2x80x3 is hydrogen, at least one of q10, q11, R9, and R10 is hydrogen. Preferably wherein q10, q11, q12, q13, rxe2x80x3, R9, R10 each R11, and each R12 are independently selected hydrogen, methyl, and ethyl.
A cyclic organic moiety having any of the formulas, (I), (II), (III), and (IV) can be introduced into the polymer through the diol. In certain embodiments, the oxygen scavenging polymer can be the product of a reaction further comprising cis-1,2,3,6-tetrahydrophthalic anhydride, in addition to the maleic anhydride and the at least one diol. When this is the case, the oxygen scavenging polymer comprises a cyclic organic moiety having formula (IV), as described above, wherein h is 1, j is 0, and wherein q10, q11, q12, q13, rxe2x80x3, R9, and R10 are hydrogen. This oxygen scavenging polymer can comprise additional cyclic organic moieties introduced into the polymer through the diol having formula (I), (II), (III), or (IV). Preferably this oxygen scavenging polymer that is the reaction product of maleic anhydride and at least one diol has an acid number of between about 5 mg KOH/g and 50 mg KOH/g of oxygen scavenging polymer.
Oxygen scavenging compositions of the present invention are thermoset, and can be useful in certain contexts in which thermoplastic compositions have limited utility. For example, certain thermoset compositions of the present invention can be used as adhesives to join articles or layers together. They can also be particularly useful as coatings for metal containers (e.g. cans).
Certain ordinary (e.g. non-oxygen scavenging) thermoset compositions can be used to treat the surface of a can to protect it from corrosion. Corrosion of a metal food can can cause food contained in it to take on an unpleasant flavor. Metal corrosion can be a particular problem with certain foods that are relatively reactive with metal and that are packaged in metal cans (e.g. carbonated beverages and acidic juices). When certain compositions of the present invention are applied to the inner surfaces of cans, they can help prevent the metal from being corroded, while providing the capacity to scavenge oxygen, thus helping to preserve the oxygen sensitive goods in the container and prolonging their potential shelf life. Increasing a good""s shelf life can provide cost savings, in reducing the need to restock and in decreasing waste.
A general advantage of thermoset materials is that they tend to inherently possess good barrier properties (e.g. gas barrier and moisture barrier properties), and certain of the thermoset compositions of the present invention have enhanced barrier properties, because of their added ability to scavenge oxygen. Certain compositions of the present invention could be used to in the field of electronics to protect oxygen sensitive electronic components.